(a) Field of the Invention
This invention relates to a light quenchable thyristor device which can be utilized in a device for converting a direct current or voltage of a large electric power to an alternating current or voltage.
(b) Description of the Prior Art
It is already well known that a thyristor device can be driven by light and is extensively practiced in the name of an LASCR or a Light Activated Thyristor. It is generally said that, in a large electric power converting device which uses a thyristor, the perfect separation of the large electric power part and control circuit from each other is realized by making the thyristor operatable from a light trigger. FIG. 1 shows the structure of the light triggering part of a thyristor having a conventional pnpn structure. The n.sup.+ region 1 represents a cathode, p region 2 represents a first base, n region 3 represents a second base) the p.sup.+ region 4 represents an anode, 6 represents an anode electrode and 9 represents a cathode electrode. The n.sup.30 region 7 represents cathode region of an auxiliary thyristor and is formed to be shallow in a part in order to increase the number of pairs of electrons and positive holes generated by the light triggering pulse passing through the light fiber cable 8 so that the light permeability may be improved. The electrode 5 is an electrode for short-circuiting the n.sup.+ cathode 7 and first base layer 2 with each other. The electrode 9 also short-circuits the n.sup.30 cathode 1 and first base layer 2 with each other in a part not shown in the drawing. Both of the n.sup.+ region 7 receiving the irradiation of the light triggering pulse and n.sup.+ region 1 are cathode regions of the thyristor shown in FIG. 1. It is shown in this thyristor that the cathodes 7 and 1 are short-circuited with the first base 2. By thus short-circuiting the n.sup.+ cathode regions 7 and 1 with the first base layer 2, the surface potentials of the n.sup.+ cathode regions 7 and 1 and of the p base region 2 are kept the same. The light triggering operation shall be explained in the following.
By the base resistance voltage drop until the carriers generated by the light flow through the base layer 2 and reach the base electrode 5 or 9, a potential distribution is produced within the base layer 2. The part through which the electrons of the cathodes 7 are most likely to flow is the base layer part just below the n.sup.+ cathode region 7. The electrons slightly flowing out of the n.sup.+ cathode region 7 flow out also to the anode 4 side together with the recombination within the base 2 and run through the n region 3 to be accumulated near the np(.sup.+) junction between it (3) and the anode p(.sup.+) region 4. Thereby, positive holes are injected into the n region 3 from the anode p(.sup.+) region 4 and the electrons flow mostly through the first base layer 2 and reach the base electrode 5 or 9. Thereby, the base resistance voltage drop within the base layer 2 is further produced and further more electrons flow out of the n.sup.+ cathode region 7 receiving the light irradiation and further the n.sup.+ cathode region 1 not receiving the light irradiation until the thyristor is turned-on. A constant resistance is inserted between the n.sup.+ region 7 and base layer 2 so as to compensate any misoperation produced at the time of triggering the light in the thyristor but, in the operation of the conventional type light triggering thyristor, utilizing the base resistance drop within the first base layer 2 is a fundamental operation. However, in turning-off, the voltage of the anode and cathode is reversed by using an electrically commuting circuit or a structure of turning-off the gate wherein parts for electrically turning-off the gate are integrated on the same chip is generally utilized.
On the other hand, the light triggering operation of a static induction thyristor (SIThy) different in the operation principle from the above described thyristor by the base resistance control and turning-on and -off by controlling the potential barrier within the channel by the static induction effect has been already suggested by the present inventor and is disclosed in Japanese patent applications Nos. 95585/1976 (laid-open No. 20885/1978) and 150300/1982 (laid-open No. 40576/1984). A method wherein a gate circuit including a photosensitive element is inserted between the first gate and cathode of the conventional static induction thyristor and the static induction thyristor is turned-off by the light irradiation to this photosensitive element has been also already suggested and is disclosed in Japanese patent application No. 36079/1979 and laid open No. 128870/1980. As compared with the thyristor of the conventional type pnpn structure shown in FIG. 1, the static induction thyristor has features that, as the potential barrier control within the channel is utilized, the frequency is not limited by the base resistance, the speed can be easily made high and the area can be made large and, as the carriers run at a high speed through the high resistance channel region, even the forward voltage drop is so small as to be less than 1.6 V at the current density of 10.sup.3 A/cm.sup.2 and further has an operation of sucking out at a high speed the carriers within the channel to the gate electrode by the drifting electric field existing between the gate and channel at the time of turning-off and therefore high speed turning-off is possible.
The already practiced or suggested matters regarding the light triggering or light quenching operation shall be summarized as follows. In the conventional type pnpn structure thyristor or gate turn-off thyristor, the light triggering operation is carried out as explained in FIG. 1 but the light quenching operation by the gate is not carried out. Generally, turning-off is electrically made by commutative circuit inserted between the anode and cathode. Regarding the static induction thyristor, only with respect to a single gate structure, the light triggering is disclosed in the above described Japanese patent application Nos. 95585/1976 and 150300/1982 and the light quenching is disclosed in the above described Japanese patent application No. 36079/1979.
However, in the embodiment disclosed in the Japanese patent application No. 36079/1979, the light pulse driving the single-gate type SI thyristor is not irradiated directly on the thyristor but is irradiated on the photosensitive element in the external circuit connected to the gate and the quenching light pulse drives the external circuit. That is to say, the impedance of the photosensitive element of the external circuit is varied with the light pulse and thereby the current source voltage applied to the gate of the SI thyristor is varied to be strong or weak to thereby trigger or quench the light in the SI thyristor. On the other hand, there is also a method wherein, in the conventional type pnpn structure thyristor, a pin photodiode as a kind of commutative circuit is connected between the anode and cathode and the light triggering operation is carried out by irradiating the triggering light directly on the thyristor as in FIG. 1 and the light quenching operation is carried out by irradiating the quenching light on the pin photodiode connected between the anode and cathode. This method is published by P. Roggwiler et al in the International Electron Devices Meeting, 1980 on p. 646. However, in this method, the area of the light quenching photodiode must be made so large as to make the anode current flow, the speed is comparatively slow and the efficiency is also low.
In the conventional type pnpn structure thyristor or the gate turn-off thyristor, the reason why the light quenching operation by the optical gate turn-off process is not made is thought to be because the time constant of turning-off becomes long due to the base resistance within the first base layer but is due to the great defect that, as the carriers accumulated in the junction part of the second base layer 3 and anode region 4 are vanished by their flow out into the anode region 4 or their recombination with the positive holes injected from the anode region 4, the time constant of turning-off becomes long. This is the same also in the light quenching operation of the static induction thyristor of a single gate structure. Further, the presence of a base resistance within the first base layer in the thyristor of the conventional type pnpn structure substantially reduces the sensitivity to the light. In the case of the light triggering and quenching operation by connecting the external circuit which includes the light sensitive element between the first base 2 and cathodes 7 and 1 and controlling the impedance of the external circuit with the light, the internal impedance of the thyristor represented by the base resistance becomes a factor of remarkably reducing the efficiency of the light triggering and quenching operation. Therefore, it is thought that, for the reason that the gate resistance is very small, the static induction thyristor is better in the light triggering and quenching sensitivity. It has not been industrially practiced to turn-on or -off the direct current with only the light. In order to perfectly separate the high power and control circuits from each other with the light, the direct current must be turned-off with the light. In the conventional type pnpn structure thyristor or the gate turn-off thyristor, the current is electrically turned-off using the commutative circuits and, in the single gate type static induction thyristor, too, the time constant of turning-off is determined by the time constant of vanishing the carriers accumulated between the second base and anode the same as in the light quenching operation. The time constant of turning-off of the light triggered thyristor of the conventional pnpn structure is comparatively so long as to be several hundred .mu. sec.